1. Field of the Invention
Aspects of this invention relate generally to cooling of Light Emitting Diodes (LEDs) when powered under electrical bias, and also to cooling the LEDs while outdoors in direct sunlight in the OFF state with no electrical bias, and more particularly to cooling of a plurality of LEDs used for outdoor lighting under conditions of solar heating or electrical bias.
2. Description of Related Art
LEDs are light emitting solid state devices comprised of semi-conducting materials joined at a junction. Electrical current through such a solid state device will produce heat, and heat build up within LEDs will not only reduce their light output but will also cause a shift of electrical parametric characteristics. However, properly designed support circuits will anticipate or track and adjust or compensate for parametric changes within certain limits. The usable or functional life of an LED is inversely proportional to its life-long average junction temperature, and if temperature exceeds prescribed limits, permanent damage to the LED will certainly occur. It is therefore important to keep the LED as cool as possible for maximum life span and ultimately to prevent or forego failure.
Ambient air heat exchangers are among the most commonly used methods to cool a semiconductor, having multiple fins to enlarge the effective surface area for convection cooling. A plurality of high-output LEDs combined in arrays will experience mutual heating and can require about nine or more square inches of surface area per watt of electrical power dissipated in order to keep the LED junctions within the safe operating temperature range specified by the LED manufacturer. Therefore an array that requires forty watts (40 W) of electrical power can require almost four hundred square inches (400 in2) of heat dissipating surface area to achieve adequate cooling in ordinary airflow. Actual LED measurements with the exemplary embodiment of the present invention with forty watts (40 W) applied have shown temperature rise for a four hundred square inch (400 in2) heat exchange area to be twenty-nine degrees Celsius (29° C.), thus providing 0.75 degrees Celsius per watt (0.75° C./W) thermal resistance to the surrounding air.
Solar heating of space and liquids is a well known science today, and solar collectors constructed of known materials collect solar energy predictably based on the known relationships of collector surface area and coating or finish, collector material, BTU rate, watts and thermal resistance. The delivery rate of solar energy used in collector calculations is 320 BTU/hour per square foot area of flat surface solar collector perpendicular to the solar rays at sea level. One BTU per hour is equivalent to 0.29 watts of power.
A typical heat exchanger of the prior art adapted to an LED luminaire will have its entire upper surface area generally perpendicular to the sun's rays, while the majority or all of the underside area will be for LED light emission and not used for cooling. With the prior art concepts, certain conditions can occur where the LED temperature can exceed eighty-five degrees Celsius (85° C.), a level that most LED manufacturers list as a maximum storage temperature not to be exceeded.
In the exemplary embodiment, the present invention is used with a relatively high lumen output device specifically intended for outdoor illumination applications, and when powered, the associated LEDs rely on a large area heat exchange mechanism for adequate cooling. Consequently, the heat exchanger is exposed to direct sunlight during daylight hours when the LEDs are not powered on. It will be obvious to one skilled in the art that a primary objective of the present invention is to reduce solar heating of the LEDs even when in the OFF state, as accomplished in the present invention, at least in part, by lowering temperature as a result of the large convection cooling mechanism exposed in the outdoor solar environment. Surprisingly, the results of actual temperature measurements and comparisons of the present invention show a better than anticipated daytime cooling effect by adding convection fin area while not simultaneously adding to the solar collecting area.
In the prior art, U.S. Pat. No. 5,782,555 to Hochstein cites temperatures that reach eighty-five degrees Celsius (85° C.) in traffic signals and discloses an LED device that operates accordingly, yet with the caveat of shortened LED life. Additional prior art U.S. Pat. Nos. 6,450,662, 6,527,422 and 6,614,358 to Hutchison each disclose solar louvered external air cooled heat sinks, thereby shielding the cooling apparatus from solar energy. These prior art inventions are evidence of the benefits and efforts to shade heat exchangers from solar energy. However, these inventions are predominantly for small signaling and display LEDs under electrical bias for traffic signals. Outdoor illumination using LEDs is relatively new technology at the time of the present invention, requiring high-power, high-output LEDs with considerable size heat exchangers that would be yet even larger if applying techniques taught by the inventions of the prior art. The preferred embodiment of the present invention reduces the solar collecting effect of said heat exchangers, and does so as a single device without the need for an additional sun shade element.
Aspects of the present invention are then directed to one or more features including but not limited to: (1) providing a single extruded component with the necessary surface area to adequately cool the LED array when operating under full electrical bias; (2) incorporating additional cooling fins that are inherently shaded from solar energy by position alone, and therefore expel or radiate a portion of the solar energy collected from the total of all surfaces without themselves collecting additional solar energy; (3) providing an LED mounting surface that has some feature of ambient convection and that can be molded or extruded to aim outward or downward at an angle not fixed by the thermal design of the other convective surfaces; (4) having the combined solar-exposed and solar-shaded cooling fins join together for balancing or limiting the solar thermal rise with some thermal resistance to the LED mounting area; and (5) forming the shape entirely as an extruded or molded component without requiring additional components to shade the cooling fins. Aspects of the present invention fulfill these needs and provide further related advantages as described in the following summary.